1. Field of the Invention
The present invention relates to an E-PON (Ethernet Optical Passive Network) including a single OLT (Optical Line Terminal) and a number of ONUs (Optical Network Units), and more particularly to a bandwidth-request transmission method for data transmission of ONUs in the E-PON structure, and DBA (Dynamic Bandwidth Allocation) technology using the method.
2. Description of the Related Art
Although MAC (Medium Access Control) technologies for ATM-PON (Asynchronous Transfer Mode—Passive Optical Network) and Ethernet have already been standardized as described in IEEE 802.3z and ITU-T G983.1, functions required in the PON structure have not been defined for the Ethernet. Among PON types, the ATM-PON has been initially developed and standardized, where upstream and downstream transmission is performed with a frame including ATM cells combined into a predetermined size, and an OLT suitably inserts downstream cells to be distributed to ONUs into this frame in a tree-based PON structure. A TDM (Time Division Multiplexing) scheme is employed to gain access to data of the ONUs in upstream transmission, while a so-called “ranging” method is used as a solution to avoid data collision in an ODN (Optical Distribution Network), a passive element. To this end, upstream and downstream frames each have a field in a general ATM cell or a dedicated ATM cell to allow exchange of messages at desired intervals.
Although ATM technology employs relatively-high priced equipment, has restrictions on bandwidth, and further requires IP-packet segmentation, subscribers demand increasingly large amounts of bandwidth with the development of Internet technologies. It has therefore become a major aim to implement end-to-end transmission through a Ethernet network which employs relatively-low priced equipment and can guarantee high bandwidth. A need exists for an Ethernet scheme rather than an ATM scheme for a subscriber network, even if the network is a PON.
In the ATM-PON, upstream and downstream frames are each composed of ATM cells of a desired size, and a TDM method is employed in upstream transmission for the tree structured point-to-multipoint connection. In the case of the Ethernet, a point-to-point or collision-based MAC protocol has already been standardized with an MAC control chip thereof commercially being available. However, although the IEEE8023.ah EFM (Ethernet in the First Mile) study group has proposed a PON structure for the Ethernet that incorporates a new point-to-multipoint structure not handled in a conventional Ethernet scheme, standardization of the overall scheduling procedure including an MAC control is currently under development and has not yet been established. Further, the current E-PON has no embodied functions as required in the PON structure.
DBA scheduling of data transmission for ONUs in the ATM-PON is performed based on their service classes to guarantee QoS (Quality of Service). For this purpose, the ONUs each have for incoming traffic data four independent queues according to their service classes defined based on parameters such as VBR (Variable Bit Rate), CBR (Constant Bit Rate), and real-time property. However, in contrast to ATM, E-PON has no defined service class because it is based on an Ethernet protocol.
E-PON bandwidth allocation also differs from that of the ATM, because the Ethernet technology underlying E-PON uses variable packet size rather than ATM cells of fixed packet size.
Because they correspond to broadcast signals of the conventional Ethernet, E-PON handles the downstream traffic signals traveling from an OLT to ONUs in the same manner. However, data collision is a problem in the upstream side, due to the point-to-multipoint PON structure. To avoid data collision caused by the inevitable multiplexing of the upstream traffic signals to transfer them to the single OLT, the OLT of a E-PON is burdened with performing time distribution for each of the ONUs so that the ONUs transmit data at different times.
FIG. 1 shows the configuration of a conventional E-PON. The E-PON includes a single OLT 110, a number of ONUs 130A, 130B and 130C, and an ODN (Optical Distribution Network) 120.
The OLT 110 is positioned at the root of a tree structure, and plays a primary role in providing information to each subscriber in an access network.
Having a tree topology structure, the ODN 120 distributes downstream data frames incoming from the OLT 110 to the ONUs 130A to 130C, and multiplexes upstream data frames for transmission to the OLT 110.
A number of the ONUs 130A to 130C receive the downstream data frames and provide them to end users 140a, 140b, 140c, and transmit data inputted from the end users 140a to 140c as upstream data frames to the ODN 120. The end users 140a to 140c include various kinds of terminal devices, such as an NT (Network Terminal), usable in the E-PON.
FIG. 2 illustrates a dynamic bandwidth allocation procedure of the OLT 110 shown in FIG. 1. As a signal for coinciding cycle periods between the OLT 110 and the ONUs 130A to 130C, a synchronizing signal “Sync” is periodically transmitted downstream from the OLT 110. The OLT 110 transmits grant frames respectively to a number of the registered ONUs 130A to 130C to give them a chance to request bandwidth allocation. When given the bandwidth allocation request chance, the ONUs 130A to 130C each transmit upstream a bandwidth allocation request frame that includes the size of a buffer in which data currently awaiting transmission is stored, at a time of starting a bandwidth allocation request in the next period. Given a chance to perform a bandwidth-allocation-requested upstream transmission, each of the ONUs 130A to 130C subsequently transmit the data awaiting transmission during its transmission time period. Thus, in comparison to the ATM-PON employing cells of a specific form, the E-PON inevitably has m ore technical constraints to constituting upstream and downstream frames or to securing QoS in TDM-based upstream transmission.